Many orthopedic implants have a coating on their surface to enhance osseointegration. These include, but are not limited to, hip, knee, shoulder, spine, extremities, and dental implants. These implants are typically made from stainless steel alloys, cobalt-chrome alloys, titanium alloys, or polymers such as polymers in the polyaryletherketone family (e.g, polyether ether ketone (PEEK), polyetherketone (PEK), polyetherketoneketone (PEKK), polyetheretherketomeketone (PEEKK) and polyetherketoneetherketoneketone (PEKEKK)), and polyethylenes. Historically, many implants were cemented in place using Polymethyl methacrylate. Today, surgeons generally prefer to rely on osseointegration to hold implants in place.
As an example, today hip implant stems are typically a composite structure consisting of either a cobalt-chrome alloy or titanium alloy substrate that carries the patient's weight, with a textured surface coating composed largely of peaks and valleys to aid in immediate fixation and ultimately promote and facilitate long term osseointegration. See FIG. 1.
Furthermore, there are many PEEK implants which are used for spinal procedures. To enhance the osseointegration of the PEEK, companies have introduced titanium plasma spray coatings to the PEEK implants.
Today, the majority of implant coatings are textured coatings, which are applied by hot plasma spray, vapor deposition (chemical and/or physical), or by sintering fiber mesh or beads. See FIG. 2.
These coatings processes leave a largely two-dimensional structure for the bone to grow around. There is no means for the bone to tunnel further into the coating so as to establish significant three-dimensional osseointegration. This may stifle or compromise long-term osseointegration. Additionally, the largely two-dimensional structures created using these technologies do not closely mimic the structure of trabecular bone, which is a three-dimensional structure with interconnecting networks of pores having capillarity properties. See FIG. 3.
Recently, there have been advances in the creation of porous coatings that more accurately resemble trabecular bone. These porous surface coatings have interconnecting networks of pores which are similar to those of trabecular bone, and may serve to promote bone ingrowth deeper into the porous coating and provide better long-term implant fixation. There are several techniques known in the art for creating these porous coatings. One method is to coat a structure similar to trabecular bone, e.g., a polyurethane foam, with powdered titanium through low temperature arc vapor deposition (LTAVD) or chemical vapor deposition or sputtering, and then to sinter the resulting structure onto the substrate (e.g., the hip implant). See FIG. 4.
Other methods include chemical vapor deposition of commercially pure tantalum onto a porous carbon scaffold and then sintering the resulting structure onto the substrate (e.g., the hip implant). See FIGS. 5 and 6.
In addition to these porous metal constructs being used as coatings, they can also be used as standalone implants. By way of example but not limitation, these porous metal constructs can serve as bone void fillers, cement spacers, femoral and tibial cone augments, buttresses, cages, spine interbody fusion devices and other bone augmentation devices including bone wedges such as Cotton and Evans wedges.
Often these standalone implants find utility in revision procedures following a primary implant failure. Removal of the primary implant leaves a large void that needs to be filled. These porous metal implants can be used to fill these voids and provide a support structure for the new implant.